In the solid-state image pickup device as the conventional CCD (charge coupled device) type image pickup device or CMOS (Complementary Metal Oxide Semiconductor) type image pickup device, incident light is converted into electrical charge by the photoelectric conversion section arranged inside a pixel, and a video image is obtained. The incident light is converted into the electrical charge, whereby a picture signal is obtained. A photo-MOS gate and a photodiode are used as the photoelectric conversion section, for example. Both the CCD type image pickup device and CMOS type image pickup device are designed to store the electrical charge produced by the incident light for a predetermined exposure time. After the lapse of the time, signals corresponding to the amount of the stored electrical charge are outputted.
There is a limit to the amount of the electrical charge that can be stored. High-intensity incident light will cause the electrical charge to be saturated, and an appropriate output signal cannot be obtained. To prevent this, a restriction is imposed on the incident light by an optical device such as a filter and aperture. In this case, the amount of the electrical charge having been stored will be too small for the low-intensity incident light, and the output signal will be hidden among the surrounding noise, with the result that appropriate output signal cannot be obtained.
Thus, a solid-state image pickup device has an acceptable range of the amount of incident light (dynamic range). This range is generally very small, as compared with the wide dynamic range of the object to be imaged. Such being the case, there has been an intense demand for expansion of the dynamic range in a solid-state image pickup device.
In the conventional art, some attempts have been made to expand the dynamic range from the aspect of the solid-state image pickup device or signal processing. One of the attempts is exemplified by the technique wherein the electrical charge produced in the photodiode is discarded from a storage device in response to the amount of light, thereby avoiding saturation due to a great amount of incident light (e.g. Non-Patent Document 1). Another example is the technique wherein charging and discharging are repeated during exposure to get the amount of signal from the number of charging and discharging operations (e.g. Non-Patent Document 2).
In a further example, the front of the image pickup surface of the CCD type image pickup device is provided with a filtering device such as a transparent liquid crystal panel where there is a change in light transmittance. The light transmittance of the filtering device is controlled in response to the level of the picture signal outputted by the image pickup device, whereby the incident light per se is controlled to expand the dynamic range (e.g. Patent Document 1).
A still further example is given in the following proposal, although this is not intended to expand the dynamic range. Namely, a light shield mask for shielding a part of the aperture of each pixel is provided on the pixel of the image pickup device. The position of the light shield mask is changed using a micro-actuator and the resolution is improved by a plurality of image pickup operations, without having to increase the number of the pixels, in this proposal (e.g. Patent Document 2).    [Non-Patent Document 1]
Fritz J. Kub and Gordon Wood Anderson, “Compressing Photodetectors for Long Optical Pulses Using a Lateral Blooming Drain structure,” IEEE Transactions on Electron Devices, vol. 40, No. 10, pp. 1740-1744, 1993.    [Non-Patent Document 2]
David Stoppa, Andrea Simoni, Lorenzo Gonzo, Massimo Gottardi and Gian-Franco Dalla Betta, “Novel CMOS Image Sensor With a 132-dB Dynamic Range,” IEEE Journal of Solid-State Circuits, vol. 37, No. 12, pp. 1846-1852, 2002.    [Patent Document 1]
Unexamined Japanese Patent Application Publication No. H6-70225    [Patent Document 2]
Unexamined Japanese Patent Application Publication No. H8-163449
According to the method disclosed in the Non-Patent Document 1, however, noise is produced from the transistor used to control the amount of electrical charge to be discarded, with the result that a satisfactory picture signal may not be obtained. According to the method disclosed in the Non-Patent Document 2, an analog-to-digital conversion circuit is required for each pixel of the image pickup device. This will result in a reduction in the exposure area ratio of the pixel and an increase in the pixel size.
According to the method disclosed in the Patent Document 1, the front of the CCD type image pickup device is provided with a filtering device such as a transparent liquid crystal panel where there is a change in light transmittance. Due to a low transmittance at the transparent state (e.g. 0.5 or less in the liquid crystal panel), the contrast of the captured image is reduced, and a complete light shielding cannot be achieved at the black state (transmittance of about 0.01 at the black state). A separate mechanical shutter is required when the electrical charge is transferred. Such a problem is found in this Patent Document. Further, there is a big characteristic change in the material of the liquid crystal panel and others due to temperature, and therefore, a further restriction is imposed on the performances at a low temperature or a high temperature. Moreover, the response speed of the liquid crystal panel and others is in the order of several milliseconds. This results in a low transmission/shielding switching speed. Such being the case, adequate restriction on the amount of light cannot be imposed in the face of the abrupt change in the amount of light.
In the method of Patent Document 2, the aperture position is switched in chronological order by the change in the position of light shield relative to the aperture of the pixel. A high resolution is merely achieved by this arrangement. Patent Document 2 does not provide variable control of the amount of the light blocked, and fails to expand the dynamic range.